This apparatus relates to a detector system for determining thermal conductivity of fluids in a well bore. A sonde suspended on a logging cable is lowered to the bottom of a well. The sonde encounters various fluids in the well bore. Identification of the fluids is enhanced by determining the thermal conductivity of the fluids and, hence, fluid identification can thereby be obtained. Typically, thermal conductivity is measured in BTU/Hour/Foot Square/Degree F/Foot and for water is 0.3263 at 72.degree. F. For transformer oil, it is 0.103 and for kerosene, it is 0.086. The thermal conductivity for methane gas is 0.0175 and for air, it is 0.014. It can be understood on noting these values of thermal conductivity that the fluid media surrounding the sonde determines the rate of heat transfer away from the sonde.
It is, however, difficult to measure thermal conductivity of the fluids adjacent to the sonde because the ambient temperature increases more or less as a function of depth ignoring geothermal wells. Accordingly, wells as deep as 15,000 to 20,000 feet are quite hot. Because of the change in ambient temperature, measurements based on relative temperature are difficult to obtain, and such temperature measurements cannot be easily isolated or separated in thermal conductivity measurements. The apparatus that this disclosure sets forth a thermal conductivity probe adapted to be incorporated in the sonde. When the sonde is lowered into the well, it is exposed to ever increasing ambient temperatures. This apparatus utilizes a resistor bridge having four legs. Two of the legs are fixed. Two of the legs include resistor elements which are both physically mounted so that they are exposed to the well fluid. Thus, the two legs exposed to well fluid will commonly drift as a function of temperature. That is, the resistance in each leg will be different as a function of the ambient temperature. Since the same ambient temperature drift is observed in both legs, the drift in the output voltage of the resistor bridge is nil and does not drift as a function of ambient temperature. This then enables the resistor bridge to obtain measurements relating to thermal conductivity.
Thermal conductivity can thus be determined by observation of down hole bridge differential voltages. Moreover, transitions between fluid media in the well bore can be observed. For instance, assume that the device of this disclosure is lowered into a well bore which has a standing column of liquid. Assume that the column is oil from the formation which has separated and which stands on top of a column of water. A sonde supporting a thermal conductivity probe in accordance with the teachings of this disclosure, if lowered in the well bore, will detect the oil and water interface in the column of liquid standing in the well. Thus, the measurement of thermal conductivity will deflect significantly on crossing the interface between oil and water. It will also deflect significantly on crossing the interface between the oil and air or methane above the oil. Such changes can be observed and hence the location of oil and water in the well bore can be determined. Moreover, thermal conductivity of mixtures can also be determined.
With the foregoing in view, the present apparatus is described a thermal conductivity probe adapated to be lowered into a well bore hole for measurement of thermal conductivity of fluids in the well, including both gases and liquids. The device is adapted to be exposed to temperature gradients in the well and yet provides an output reading which is not altered by the ambient temperature variation. Many advantages flow from use of the thermal conductivity probe incorporated in a sonde as set forth in this disclosure.